A technology disclosed in this specification relates to a semiconductor device having a wiring board with a semiconductor element mounted thereon and to a method for fabricating the same.
Size reduction of electronic devices in recent years intensely demands size reduction and higher packaging density of semiconductor devices installed in the electronic devices. To meet the demand, the form of the semiconductor devices has shifted from conventional insertion type, such as DIP (Dual Inline Package), to surface-mount type, such as SOP (Small Outline Package) and QFP (Quad Flat Package). The package forms more widely adopted in recent years with the aim of achieving advanced size reduction and higher packaging density are a package called BGA (Ball Grid Array) where external terminals are arranged in an array over a whole face of the lower part of the device, and a package called CSP (Chip Size Package) where external terminals are arranged with shorter intervals than BGA and the package size is smaller than BGA. The essential features and fabrication methods of BGA and CSP are described below.
The wiring board generally employed for BGA and CSP is a multilayered wiring board including an organic material. The multilayered wiring board includes a wire part formed of a metal material, such as Cu, and an insulator part formed of, for example, a resin or glass cloth. Mounting of a semiconductor element on the wiring board is generally achieved by wire-bonding but, in some cases, by flip-chip mounting with the aim of improving electrical properties. The semiconductor element mounted on the wiring board may be overmolded with a resin when necessary. Also, in general, the back surface of the wiring board (the face opposite to the surface with the semiconductor element mounted thereon) is provided with solder balls which serve as the external terminals.
A method for fabricating BGA and CSP is now described. Fabrication of BGA and CSP employs a wiring board plate for improvement in production efficiency which is to be separated into a plurality of wiring boards carrying semiconductor elements mounted thereon. Specifically, the semiconductor elements are mounted at predetermined positions on the wiring board plate and, when necessary, the semiconductor elements are overmolded, or solder balls are fixed over the back surface which serve as external terminals. Then, in the final step, the wiring board plate is cut along a wiring board perimeter line into individual wiring boards. The two typical perimeter-cutting methods described below are known in the art.
The first method is dicing with a diamond blade, or the like, for separation into semiconductor devices. This method uses a diamond-coated file blade for perimeter cutting and, therefore, a resultant semiconductor device having an improved cut surface roughness is produced. Dicing however needs to be carried out along the perimeter lines of the respective semiconductor devices, which is disadvantageous in respect of workability and mass-productivity. Also, to prevent degradation of semiconductor device edges due to frictional heat from the dicing saw, spraying of water, or the like, over the dicing saw is necessary. In this case, the semiconductor element mounted on the wiring board is exposed to water, and therefore, there is a concern that the semiconductor element absorbs moisture, resulting in addition of drying the semiconductor devices after perimeter cutting.
The second method consists of loading the wiring board plate via registration holes formed in the wiring board plate in advance to fix the position of the wiring board plate and then punching the wiring board plate using a metal mold along the perimeter lines for separation into semiconductor devices. This method is superior to the first method in respect of workability and mass-productivity and is as of now the most popular among the existing ones. A commonly-employed method example of punching the wiring board plate with a metal mold is a combination of elongated hole formation with a drill called a router and punch stamping. A conventional wiring board fabrication method based on the combination of elongated hole formation and punch stamping is described below with reference to FIGS. 12 to 16.
FIGS. 12A to 12C are plan views illustrating a conventional wiring board fabrication method. FIG. 13 is an enlarged plan view showing a corner of the perimeter line of a semiconductor device region during punch stamping. FIG. 14 is a plan view of a conventional wiring board after fabrication.
Referring to FIG. 12A, the first step is forming registration holes 102 in a wiring board plate 101 and holes 104 at the corners in a perimeter line 103 of respective semiconductor device regions (wiring board regions) defined over the wiring board plate 101. Note that the holes 104 are for making rounded surfaces at the corners of the semiconductor device. Then, elongated holes 105 are formed in the wiring board plate 101 using a router so as to extend between adjacent holes 104. Each of the elongated holes 105 is formed such that the inner longitudinal side of the elongated hole 105 is coincident with the perimeter line 103 of the semiconductor device region. Note that the length of a portion between an end of the elongated hole 105 and the hole 104 (unremoved portion) is set so as to secure such a strength that the wiring board plate 101 survives subsequent device assembly steps without being broken in the unremoved portion.
Referring to FIG. 12B, semiconductor elements 106 are then mounted on the resultant wiring board plate 101 at predetermined positions in the wiring board regions. The mounted semiconductor elements 106 are then overmolded with sealing resin 107 as shown in FIG. 12C. Thereafter, although not shown, for example, solder balls are fixed over the back surface of the wiring board region which serve as external terminals.
Referring to FIG. 13, the unremoved portions between the ends of the elongated holes 105 and the holes 104, which connect the semiconductor device regions to the frame of the wiring board plate 101, are then punched out to isolate the semiconductor device regions into semiconductor devices. In this way, the wiring boards are fabricated.